undefended adj.

prefix of negation, Old English un-, from Proto-Germanic *un- (source also of Old Saxon, Old Frisian, Old High German, German un-, Gothic un-, Dutch on-), from PIE *n- (source of Sanskrit a-, an- "not," Greek a-, an-, Old Irish an-, Latin in-), combining form of PIE root *ne- "not." Often euphemistic (such as untruth for "lie").

The most prolific of English prefixes, freely and widely used in Old English, where it forms more than 1,000 compounds. It underwent a mass extinction in early Middle English, but emerged with renewed vigor 16c. to form compounds with native and imported words. It disputes with Latin-derived cognate in- (1) the right to form the negation of certain words (indigestable/undigestable, etc.), and though both might be deployed in cooperation to indicate shades of meaning (unfamous/infamous), typically they are not.

It also makes words from phrases (such as uncalled-for, c. 1600; undreamed-of, 1630s; uncome-at-able, 1690s; unputdownable, 1947, of a book; un-in-one-breath-utterable, Ben Jonson; etc., but the habit is not restricted to un-; such as put-up-able-with, 1812). As a prefix in telegramese to replace not and save the cost of a word, it is attested by 1936.

mid-13c., defenden, "to shield from attack, guard against assault or injury," from Old French defendre (12c.) "defend, resist," and directly from Latin defendere "ward off, protect, guard, allege in defense," from de "from, away" (see de-) + -fendere "to strike, hit, push," attested only in compounds (such as offendere "to strike against; encounter;" infensus "aggressive, hostile"), from PIE root *gwhend- "to strike, kill" (source also of Hittite kue(n)zi "to kill," Sanskrit ghnanti "to kill; Greek theino "to slay, to kill;" Armenian jnem "to strike;" Lithuanian ginti "to protect, defend;" Old Irish gonaid "wounds, kills;" Welsh gwan "to thrust, hit;" Old Breton goanaff "to punish, sting").

It is attested from c. 1300 as "fight in defense of" (someone or something). From mid-14c. as "defend with words, speak in support of, vindicate, uphold, maintain." In Middle English it also could mean "forbid, prohibit; restrain, prevent." In the Mercian hymns, Latin defendet is glossed by Old English gescildeð. Related: Defended; defending.